THE COAST MOUNTAINS BATHOLITH AT 100-MA: RECORD OF THE TRANSITION FROM SINISTRAL-OBLIQUE TO STRONGLY CONVERGENT TWO-PLATE MARGIN

The Coast Mountains Batholith (CMB) of British Columbia and northern Washington records several major changes in arc magmatism and associated upper plate deformation at ~100 Ma that represent an important shift in plate kinematics along the western margin of the northern Cordillera. Prior to 100 Ma, the CMB developed as two distinct arcs (western vs eastern) that were rooted in different superterrane basements (insular vs intermontane, respectively) and the 100-Ma event is traditionally interpreted as the terminal suturing of these terranes. New geochronological constraints on the timing of sinistral intra-arc shear zones in both the western and eastern arcs suggests that the Insular and Intermontane superterranes were kinematically linked as early as 114 Ma, when they were still separated by the intervening Gravina /Tyaughton-Methow basins. Employment of a new large-n igneous zircon U-Pb method more than doubles the precision of measurements by LA-ICP-MS (from ~1% to 0.4%) and allows us to demonstrate the close temporal-spatial relationship between magmatism and deformation by dating comagmatic cross-cutting phases. Crystallization ages of pre-, syn-, and post-kinematic intrusions shows that the intra-arc shear zones record an Early Cretaceous phase of sinistral oblique convergence along the western margin that terminated between 107 and 101 Ma. Shear zone cessation coincides with: (1) collapse of the Gravina basin, (2) assimilation of a single voluminous arc that stitched the Insular and Intermontane superterranes, and (3) development of widespread contractional structures throughout the Coast Mountains. We interpret these concurrent tectono-thermal events to mark a shift in plate kinematics from a sinistral oblique system involving separate terranes and intervening ocean basins to a strongly convergent two-plate margin involving a single oceanic plate and the newly stitched-together western margin of North America.